Normal users should use the current Final version of XnView, which is version 1.70.4. Please see post XnView 1.70.4 for details.

A Release candidate (RC) is a mainly stable version, which is provided to a larger number of people for testing. If the release candidate has few or no serious bugs, it will become the new final release, which will be announced in a new, separate topic in this newsgroup. Typically, the release candidate comes as executable file, only, and has no setup.
If you want to use the latest version and/or have fun testing an application which still might have bugs, the release candidate is the right thing.